The invention concerns a method for the routine identification of the material of plastic components with the assistance of infrared spectroscopy with which an infrared reflection spectrum is taken from the surface of a plastic component under investigation and compared to a set of reference spectra, whereby the material of the plastic component under investigation is correlated to a class of plastic materials represented by one of the reference spectra.
A method of this type is known in the art of near infrared spectrometry from the article by H. Schope-Stein in the company magazine "Der Fraunhofer", March 1992, page 29 of the Fraunhofer-Gesellschaft.
With the enormous plurality of currently used plastic components, huge amounts of different kinds of plastic refuse with widely varying properties are continuously produced. In particular, for the purpose of recycling, there is therefore a substantial need for a routine method with which different plastics in refuse can at least be roughly sorted according to class and easily identified. A particularly important application area is thereby the recycling of plastics in the automobile industry.
Towards this end an infrared (IR) spectrometer in the near infrared (NIR: 1,000-2,000 nm corresponding to 4,000-10,000 cm.sup.-1) region, described in the above mentioned publication is, among others, suitable for this purpose using a fast tunable filter in connection with fast electronics.
An FTIR-spectrometer a well as a method for the taking of reflection spectra from the surfaces of samples in the infrared wave length region is known in the art from U.S. Pat. No. 5,160,826. In particular, a window for an FTIR-spectrometer is described in the publication with which a thin sample layer can be investigated, on the one hand, in the visible region in transmission with the assistance of a microscope component of the spectrometer and, on the other hand, in the infrared region under reflection.
A method and an apparatus for the determination of the physical properties of electronic components comprising semiconductor materials is known in the art from U.S. Pat. No. 5,255,070 with which a test beam of monochromatic light is directed onto the surface of a material sample, whereby the light beam is subjected to a time modulation of the electric vector by means of a modulated pump beam and, whereby the light reflected from the sample surface is detected and analysed.
In plastic identification methods which are also carried out with the assistance of Fourier transformation infrared (FTIR) spectrometers, one works solely in the NIR region since the glass optical components utilized are particularly accessable in this wave number range. In the mid-infrared range (MIR: 400-5,000 cm.sup.-1), where the window or lens materials are in general hygroscopic salts exhibiting unattractive mechanical properties, the above described method has never been applied. In addition, in contrast to the NIR range, fiber optics is not applicable in the MIR region or usable only under very difficult conditions.
Also for the case of opaque IR plastics, investigations of materials using spectrometers must be carried out in the reflection mode. However, most plastics exhibit strong absorption bands in the MIR region. The light which is reflected from the normally non-planar or rough surfaces has, in addition to a direct scattering component, a significant diffuse scattering portion and multiple reflections occur which, for their part, exhibit a large absorption component. This leads to the fact that MIR spectra of plastics are strongly distorted, exhibit unstable base lines and, in general, tend to resemble the derivative of an IR spectrum. Using such experimental spectra, it is in general not possible to reconstruct a clean "true" absorption spectrum of the relevant material such as one would obtain using absorption spectroscopy of the same substance under laboratory conditions.
These difficulties with the interpretation of the obtained spectra have caused one of average skill in the art to refrain from the consideration of routine investigations of plastics in the MIR range under reflection. With plastics filled with carbon, such as those normally present in motor vehicle parts production, the utilization of spectroscopy in the NIR range is, on the other hand, not possible since NIR spectra capable of analysis cannot be obtained with this type of plastic. For the above mentioned reasons up to this point in time, MIR spectroscopy has been considered to not be suitable as a method for the routine identification of plastics. However, at least a portion of the plastics materials of interest cannot be investigated with conventional NIR spectroscopic methods.